Slow decreases of ionic conductance across neuronal cell membranes, which generate slow postsynaptic potentials, can increase the effectiveness of synaptic transmission. Slow conductance decreases of sufficient magnitude increase the amplitude of monosynaptic fast EPSPs in B cells of the bullfrog sympathetic ganglia. By this postsynaptic mechanism, activation of one synaptic pathway can cause an increase in transmission, lasting several minutes, across another synapse. This may provide an important mechanism for synaptic integration and control of neuronal interaction.